This application is based on patent applications 2000-012568 and 2000-097091 filed in Japan, the contents of which are hereby incorporated by references.
1. Field of the Invention
The present invention relates to an actuator and a driving method thereof for moving a driven object such as a rotor or a rod by intermittently contacting a driving member to the driven object, in which the driving member is moved for trailing a circular or an elliptical trail by utilizing vibrations of at least one of a plurality of displacing elements such as piezoelectric devices.
2. Description of the Related Art
In recent years, a truss-type actuator is proposed for rotating or moving a driven object in a predetermined direction. In the truss-type actuator, two displacing elements such as piezoelectric devices are arranged to cross the displacing directions thereof at a predetermined angle such as 90 degrees. The displacing elements are respectively driven by alternating voltage signals having a predetermined phase difference so that a driving member provided at a crossing point of the displacing elements is moved for trailing a circular or an elliptical trail. When the driving member is intermittently contacted with the driven object, the driven object is rotated or moved in the predetermined direction by a friction force acting between the driving member and the driven object.
When sinusoidal voltage signals respectively having different phases are applied two piezoelectric devices, the piezoelectric devices respectively repeat expansion and contraction, so that they sinusoidally displace corresponding to the phase of the driving voltages. Thus, a chip member serving as a driving member coupled to the piezoelectric devices is moved for trailing an elliptical trail. When the phase difference of the sinusoidal displacements of the piezoelectric devices is 90 degrees, the chip member is moved for trailing a circular trail.
When the chip member is moved circularly or elliptically, the chip member is intermittently contacted with a rotor serving as a driven object. The rotor follows the movement of the chip member by a friction force acting between the chip member and the rotor while the chip member is contacted with the rotor, so that the rotor is intermittently rotated in a predetermine direction. By repeating these motions, a power of the actuator can be outputted via the rotation of the rotor.
For controlling the rotation velocity of the rotor, it is proposed to vary at least one of a frequency, a voltage and a phase difference of driving signals (sinusoidal voltage signals) applied to the piezoelectric devices. In a first conventional actuator shown in Publication Gazette of Examined Japanese Patent application Hei 7-114550, a velocity of a driven object is controlled by switching between a first velocity control for varying a voltage of driving signals applied to piezoelectric devices and a second velocity control for varying a frequency of the driving signals corresponding to a desired velocity of the driven object. The first conventional actuator, however, has a disadvantage that a variation of an electric power consumption is larger and a burden of an electric power supply becomes larger, since the moving velocity of the driven object is controlled by varying the voltage or the frequency of the driving signals.
In a second conventional truss-type actuator shown in Publication Gazette of Examined Japanese Patent application Hei 6-36673, a phase difference between two driving signals applied to two piezoelectric devices is fixed to be 120 degrees, and the piezoelectric devices are driven in a manner so that a chip member is moved to trail a compressed elliptical trail for increasing a moving velocity and a torque of a driven object. The second conventional actuator, however, has a disadvantage that the moving velocity varies corresponding to variation of a load of the actuator, since the phase difference of the driving signals is fixed.
A purpose of the present invention is to provide an actuator and a driving method thereof in which the moving velocity and the torque of the driven object can be controlled to be desired values even though the electric power consumption of the actuator is substantially constant.
An actuator in accordance with the present invention comprises: a base member; a plurality of displacing elements for generating a predetermined displacement, in which top ends of them are coupled at one point and base ends of them are respectively fixed on the base member; a pressing member for contacting the coupled top ends of the displacing elements to a driven object with a predetermined pressure; a driving circuit for applying at least one driving signal to at least one displacing elements; and a controller for controlling the driving signal so that the coupled top ends of the displacing elements moves for trailing an elliptical trail; wherein the controller controls the driving signal in a manner so that shape of the elliptical trail of the coupled top ends of the displacing elements is varied.
By such a configuration, a velocity or a torque of the driven object can be varied corresponding to the shape of the elliptical trail of the coupled top ends of the displacing elements. More concretely, a diameter of the elliptical trail in the tangential direction at the contacting point of the coupled top ends of the displacing elements and the driven object influences to the velocity of the driven object, and a diameter of the elliptical trail in the normal direction influences to the torque or driving force of the driven object. Thus, it is possible to make the velocity of the driven object faster by enlarging the diameter of the elliptical trail in the tangential direction, and to make the torque of the driven object stronger by enlarging the diameter of the elliptical train in the normal direction, and vice versa.